We´re evaluating how easy would be to implement some generic 3D interaction techniques in Crystal Space, and we wonder if you could point to us some generic information before we start our development. We have as reference the interaction techniques mentioned in "3D User Interfaces. Theory and Practice", from Bowman, Kruijff, LaViola, and Poupyrev. Could you please answer the following questions with a number between 1 and 9 (1 for difficult, 9 for easy) and a short comment.